1. Field of the Invention
The present invention relates to systems for reducing the visibility of noise in video displays.
1. Description of the Related Art
The problem of reducing the visibility of noise in television displays has been addressed for many years. Various noise reducing circuits are known. A first type of system operates on the coring principle, i.e. the incoming video signal is separated into high frequency components and low frequency components, the high frequency components containing the noise to be reduced. The high frequency component is then passed through a threshold circuit which eliminates signal components having an amplitude less than the threshold value. The resulting cored signal is then recombined with the low frequency component to provide the noise reduced video signal. An example of this type of circuitry with bidimensional noise reduction may be found in U.S. Pat. No. 4,142,211 of Yves Faroudja. This patent is hereby incorporated by reference into the present application. A good summary of the systems for noise reduction by use of coring circuits prior to 1977 may be found in that patent.
Another basic approach to solving the noise visibility problem in video displays includes storing a television field or frame, comparing the present or incoming pixel values to the corresponding stored pixel values and adding the two signals together in a ratio determined by the result of the comparison. For high differences between the present and the stored value only the present value is utilized. As the difference decreases, an increasing percentage of the stored value is used. In this type of system it is assumed that relatively large differences between corresponding points in adjacent or nearly adjacent frames or fields result from motion. In the presence of motion, noise is tolerated, while for stationary areas of the picture, a larger noise reduction takes place. An example of this type of system is taught in U.S. Pat. No. 4,107,739 to Rossi et al. Many variations of this type of circuit are known. (See, e.g. U.S. Pat. Nos. 4,107,739; 4,064,530).
Both of the above types of noise reducing circuits are based on the assumption that it is impossible or excessively difficult to isolate a true noise element from changes in pixel values resulting from motion. It is therefore possible that low level motion signals are mistakenly identified as noise and are suppressed, while high level noise signals are retained, based on the assumption that they are motion signals.
The only patent known to applicant in which an attempt is made to measure actual noise and utilize this in a noise reduction circuit is U.S. Pat. No. 4,189,755 to Balbes et. al. The noise measuring method in this patent differs from that of the present invention since no comparison between the back porch signal (or the color burst signal) from one scan line to the same signal from another scan line is carried out. Further, the measured noise signal is used as part of a signal-to noise ratio signal in response to which the bandwidth of the receiver is adjusted. This method is particularly suitable for the frequency modulated TV signals used in satellite transmissions.